As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and operating voltage, long cycle lifespan, and low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is recently increasing, research into electric vehicles (EVs), hybrid EVs (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of EVs, HEVs, and the like, a nickel metal-hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage and output stability is actively underway and some lithium secondary batteries are commercially available.
A lithium secondary battery has a structure in which an electrode assembly, in which a porous separator is interposed between a cathode and an anode, each of which includes an active material coated on an electrode current collector, is impregnated with a lithium salt-containing non-aqueous electrolyte.
In such lithium secondary batteries, charging and discharging processes are performed while lithium ions of a cathode are repeatedly intercalated into and deintercalacted from an anode. Although there are differences in theoretical capacities of batteries according to kinds of electrode active materials, in most cases, charge and discharge capacities are deteriorated as cycles proceed.
Such phenomenon is mainly attributed to non-functioning of active materials due to separation of electrode active material components or separation between an electrode active material and a current collector by change in volume of an electrode occurring as charging and discharging of a battery proceed. In addition, in the intercalation and deintercalation processes, lithium ions intercalated into an anode are unable to be properly deintercalated therefrom and thus an anode active site is reduced and, accordingly, charge and discharge capacities and lifespan characteristics of a battery are deteriorated as cycles proceed.
With regard to this, binders provide adhesion between electrode active materials and adhesion between an electrode active material and an electrode current collector and suppress volumetric expansion according to charging and discharging of a battery, which is an important factor determining battery performance.
However, when a large amount of binder is used during manufacture of a secondary battery to enhance adhesion, the amount of a conductive material or an electrode active material relatively decreases and thus conductivity of an electrode is reduced or battery capacity is reduced. In addition, if an electrode slurry is too dilute, it is not easy to perform coating of an electrode.
Therefore, there is an urgent need to develop technology of using an appropriate amount of binder and imparting high adhesion between an electrode active material and a current collector, whereby secondary battery performance may be enhanced.